These securities aid in unpacking the cash flows from a pass-through. The most uncomplicated stripped mortgage-backed securities are the PO-IO-security. Unlike a pass-through security which distributes the entire cash flows in equal proportion to all investors, the (principal only) PO-IO (interest only) creates two securities. The PO-IO aid in hedging, interest rate and prepayment risks. 

The POs get all the principal flows from each mortgage payment. They are sold at a significant discount to par. The cash flows typically are low in the beginning which increase with time as the principal component of the mortgage payment grows. The PO investor receives the entire par value of the PO .

The IOs get all the interest flows from each payment. The cash flows are very huge in the beginning and get smaller with passage of time. Therefore, they have a shorter life. The IO strips have the risk of receiving cash flows with low value than expected because the interest produced depends on the beginning-of-month balance of the pool. The IO investors may end up with cash flows in falling interest rate scenario as the mortgage rate will be paid off sooner than estimated.

Investment Characteristics of POs and IOs:

• The investment performance of POs is highly sensitive to the prepayment rates. As prepayment rates rise with a decline in mortgage rates, prices of POs increase as interest rates fall.

• The price of an IO security is positively related to the mortgage rates at low existing levels. A decline in market rates below that of the mortgage rates results in an increase in the prepayment rates and a fall in principal amounts. Interest payments to the IO fall as they are based on the principal outstanding on the underlying pool. The cash flows to the IOs improve when the interest rates increase above the contract rate.

• Unlike pass-through securities, POs and IOs demonstrate higher price volatility, since the returns on these stripped securities are negatively correlated, that is, their prices move in a direction opposite to that of the interest rates. However, their combined volatility is equal to the price volatility of the pass-through security.